In the past, atrial arrhythmias have been largely undertreated due to the perception that these arrhythmias are relatively benign. As more serious consequences of persistent atrial fibrillation have come to be understood, such as an associated risk of relatively more serious ventricular arrhythmias and stroke, there is a greater interest in providing implantable atrial or dual chamber cardioverter defibrillators for treating atrial arrhythmias. In such devices, it is desirable to synchronize delivery of a high-energy atrial cardioversion or defibrillation pulse, referred to herein as a “shock”, to a ventricle depolarization to avoid inducing ventricular arrhythmias due to stimulation during the ventricular vulnerable period.
A number of proposals have been set forth for controlling the timing of atrial cardioversion and defibrillation shocks. An early disclosure of an implantable atrial defibrillator, U.S. Pat. No. 3,952,750 issued to Mirowski et al., proposes synchronizing delivery of the atrial defibrillation pulse to a sensed R-wave. However, because cardioversion or defibrillation shocks synchronized to a ventricular rhythm that is too rapid or unstable may induce ventricular arrhythmias, more complex systems have been proposed for controlling the synchronization of atrial cardioversion with the ventricular rhythm. For example, as proposed in U.S. Pat. No. 5,207,219, issued to Adams et al., atrial defibrillation shocks are delivered synchronized to sensed ventricular depolarizations only when the ventricular depolarizations follow preceding ventricular depolarizations by an interval greater than a minimum V-V interval. In U.S. Pat. No. 5,843,130 issued to Jazayeri, a system for providing delivery of an atrial defibrillation shock synchronized only to beats during heart rate deceleration is proposed.
An implantable atrial defibrillator that applies an atrial therapy only if the ventricle is stable to insure that the therapy does not provoke a ventricular tachycardia is generally disclosed in U.S. Pat. No. 5,999,850, issued to Dawson et al. A cardiac rhythm management system that includes atrial shock timing optimization and actively stabilizes the ventricular heart rate to obtain less potentially proarrhythmic conditions is generally disclosed in U.S. Pat. No. 6,430,438 issued to Chen et al.
Commercially available ICDs typically include some method to assure that a minimum R-R interval has elapsed as a pre-requisite to delivering an atrial shock. One such method for synchronizing atrial defibrillation to a ventricular rhythm that is not too fast requires a ventricular depolarization to fall outside a defined refractory period following the immediately preceding ventricular depolarization, as in the Medtronic, Inc. Jewel® AF, Model 7250 Implantable Cardioverter Defibrillator. This method ensures that atrial defibrillation will safely be synchronized to a ventricular rate that is not too fast. However, one limitation of setting a fixed refractory period is that, if the ventricular rate is just slightly faster than the fixed refractory period, an atrial cardioversion or defibrillation therapy will not be delivered, even though the ventricular rate may be stable and a synchronized atrial therapy could be safely delivered.
Another clinical limitation of this safety feature has arisen when patient's equipped with a patient programmer attempt to initiate an atrial defibrillation therapy. If the ventricular rate is faster than the corresponding programmed refractory period required to synchronize atrial defibrillation, the atrial defibrillation that the patient is expecting will not occur, giving the patient the perception that the device is malfunctioning.
It is desirable therefore, to provide an improved method for synchronizing atrial cardioversion or defibrillation therapies to a ventricular rhythm, which allows ventricular synchronized atrial shock delivery when the ventricular rate is near the programmed limit but does not quite reach a fixed rate limit for synchronization.